1. Field of the Invention
The present invention relates to an information recording/reproduction apparatus for recording and/or reproducing information on/from an optical recording medium such as a compact disk, a photomagnetic disk and a phase change disk. Throughout the present specification, the wording "recording/reproduction" means the performing of recording or reproduction or both.
2. Description of the Related Art
Generally an information recording/reproduction apparatus includes a support driving mechanism for moving an objective along the optical axis, wherein the relative distance between the objective and optical recording medium is controlled such that a light spot formed by the objective can be located on the information recording face. In some information recording/reproduction apparatuses, an optical system is divided into a fixed section and a movable section in order to improve access and tracking. Naturally, a support driving mechanism is provided in the movable section. A support driving mechanism with a long stroke is large and heavy and thus decreases the driving sensitivity of the movable section. Therefore, a short-stroke, small-sized, and light support driving mechanism is favorable when it is loaded into the movable section. It is thus desirable that the information recording face of an optical recording medium be formed close to the focal point when the optical recording medium is loaded into the apparatus.
Recently, an information recording/reproduction apparatus for recording/reproducing information on/from a plurality of types of optical recording medium has been proposed. For example, these types of optical recording medium include a single-faced disk with only one information recording face and a double-faced disk with two information recording faces.
It is needless to say that in the above-described information recording/reproduction apparatus the optical recording mediums of different types vary in positional relationship between the information recording face and the focal point; accordingly, the reference position of the objective in focus control needs to be changed in accordance with the types of optical recording medium. To handle this problem, a long-stroke support driving mechanism is employed, which undesirably lowers the driving sensitivity of the movable section.
For a spindle motor for rotating a disk, it is desirable to fix the weight of the disk regardless of the type of disk. The simplest way of achieving the fixed weight is to vary the thickness of a protective layer (cover glass) covering an information recording face between single-faced and double-faced disks. The thickness of the protective layer of the single-faced disk is about twice as great as that of the protective layer of the double-faced disk.
An information recording/reproduction apparatus is designed by setting an ideal value for each of various parameters; accordingly, one ideal value is set for the thickness of a protective layer. For this reason, a spherical aberration is small enough so that it may be ignored with respect to an optical recording medium whose protective layer has a thickness close to the ideal value, whereas it is large enough so that it cannot be ignored with respect to an optical recording medium whose protective layer has a thickness far from the ideal value.
In order to record/reproduce information on/from optical recording mediums of different types the protective layers of which have different thicknesses by a single information recording/reproduction apparatus, it is necessary to take measures for correcting a spherical aberration caused by differences in thickness among the protective layers.
Jpn. Pat. Appln. KOKAI Publication No. 3-260934 discloses an information recording/reproduction apparatus which is applicable to a plurality of optical recording mediums of different types whose protective layers have different thicknesses. This apparatus comprises a movable section having an objective, wherein a corrective optical element is properly attached to the movable section according to the type of disk to correct a spherical aberration caused by difference in thickness among the protective layers.
However, both the weight and the center of gravity of the movable section vary depending upon whether the corrective optical element is attached or not. The variations in weight vary the driving sensitivity, and the variations in driving sensitivity vary the servo gain of a drive head. These variations are unfavorable for focus control. Moreover, the variations in the center of gravity make it impossible to maintain an ideal condition in which the driving force is always exerted on the center of gravity. As a result, a resonance occurs in the movable section thereby degrading servo characteristics.
Still further, the corrective optical element is attached or detached outside a disk, the moving distance of the movable section is greater than that in a commonly-used apparatus, and the size of the apparatus becomes larger since it requires a space for attaching/detaching the corrective optical element.